Batteries charge slower once they are almost full due to a process called "saturation" or "trickle charging." This occurs during the final stage of the charging process when the battery voltage approaches its maximum capacity.
During the initial stages of charging, the battery can accept a higher charging current as it is not yet fully charged. As the battery becomes more charged, the internal resistance increases, making it more difficult for the charging current to pass through. This increased resistance slows down the charging process.
Furthermore, as the battery reaches its maximum capacity, the charging system adjusts the charging voltage to a lower level to prevent overcharging and potential damage to the battery. This lower voltage reduces the charging current, resulting in a slower charging rate.
The purpose of this slower charging rate is to ensure the battery is charged safely and to prevent overcharging, which can lead to excessive heat generation, reduced battery lifespan, and potentially hazardous conditions.
Overall, the slower charging rate during the final stages of battery charging is a protective measure to optimize battery performance, longevity, and safety.